The Heart The muscle layer of the heart is the myocardium. Its powerful contractions pump blood through the circulatory system. The human heart has four chambers. A wall called the septum separates the right side of the heart from the left side. On each side of the septum are an upper and lower chamber.

Each upper chamber, or atrium (plural: atria), receives blood from the body; each lower chamber, or ventricle, pumps blood out of the heart.

Flaps of connective tissue called valves are located between the atria and the ventricles and between the ventricles and blood vessels leaving the heart. The valves open and close to keep blood moving in one direction.

The heart pumps blood through two pathways:

Pulmonary circulation pumps blood from the heart to the lungs and back to the heart again. Blood picks up oxygen and releases carbon dioxide in the lungs.

Systemic circulation pumps blood from the heart to the rest of the body. Cells absorb much of the oxygen and load the blood with carbon dioxide.

The heart muscle beats in an orderly and coordinated way. A small group of cardiac muscle fibers, the sinoatrial node (SA node), is also called the pacemaker. When the pacemaker fires, an electrical impulse causes the atria to contract. Another group of muscle fibers, the atrioventricular node (AV node), causes the ventricles to contract. The nervous system influences the SA node, increasing or decreasing heart rate to meet the body’s needs.

Blood Vessels Blood flows through the circulatory system in blood vessels:

Arteries are large vessels that carry blood away from the heart to the tissues of the body. Except for the pulmonary arteries, all arteries carry oxygen-rich blood.

Capillaries are the smallest vessels. Their thin walls allow oxygen and nutrients to pass from blood into tissues and wastes to move from tissues into blood.

Veins return blood to the heart. Many have valves that prevent backflow.

The contractions of the heart produce a wave of fluid pressure in the arteries, known as blood pressure. Without that pressure, blood would stop flowing through the body. The body regulates blood pressure through actions of the brain and the kidneys.

The circulatory system interacts with every organ and system in your body. The system is connected to all of your body's cells transporting oxygen from your lungs efficiently. It also carries dissolved carbon dioxide, that cells produce as a waste, back to the lungs to be expelled when you exhale.

Every cell that needs oxygen needs access to the fluids in your circulatory system. The circulatory system and its fluids are super important to your digestive system that has absorbed nutrients from your food. These nutrients are transported to all cells through the circulatory system.

The main components of the circulatory system are the heart and all the vessels. The human heart has four chambers while other organisms may have one, two, or three chambers. The heart is the pump for the circulatory system. As blood is pumped from the heart, it passes through a system of arteries, arterioles, and then capillaries. The capillaries are the vessels that allow for most of the transfer of compounds and dissolved gases. Capillaries are very small and have thin walls to allow easier passage of compounds including nutrients, glucose, carbon dioxide, and waste products. After the capillaries, your blood passes through venules and veins. The veins lead the circulatory fluids back to the heart. That's it, the system is complete (and closed).

Three common and serious diseases of the circulatory system are:

Heart disease: A leading cause of heart disease is atherosclerosis, a condition in which fatty deposits called plaque build up in artery walls and eventually cause the arteries to stiffen. A heart attack occurs as heart muscle cells become damaged.

Stroke: A clot that blocks a blood vessel in the brain may cause a stroke, which is the sudden death of brain cells when their blood supply is interrupted. A stroke can also occur if a weak vessel breaks and causes bleeding in the brain.

High blood pressure, or hypertension, is usually defined as blood pressure higher than 140/90. Uncontrolled high blood pressure can damage the heart and blood vessels. It can also lead to heart attack, stroke, and kidney damage.

Understanding Circulatory Disease Cholesterol is a lipid that is part of animal cell membranes. It is transported in the blood primarily by two types of lipoproteins: low-density lipoprotein (LDL) and high-density lipoprotein (HDL). The liver manufactures cholesterol, but it also comes from animal product foods. High cholesterol levels, along with other risk factors, lead to atherosclerosis and higher risk of heart attack.

Risk Factors

Some of these risk factors a person can't do anything about, like being older and having other people in the family who have had the same problems. But people do have control over some risk factors — smoking, having high blood pressure, being overweight, bad eating habits and not exercising can increase the risk of getting cardiovascular disease.

Tutorial: Blood Flow